Circadian variation, i.e. the periodic fluctuation of a physiological variable due to a patient's daily rhythm, is a phenomenon that affects data measured in human beings over long time periods, in particular over a period of 24 hours; see, e.g., FIG. 1. Circadian variation reflects the daily activity of the patient as compared to the nocturnal resting state, and thereby also indirectly reflects the capacity of the cardiovascular system. This suggests that parameters that describe circadian variation in a quantitative manner are potential indicators of the development or worsening of cardiac disease.
It has similarly been determined that the periods during which a patient is sleeping or awake are markedly different physiological situations for the cardiovascular system. These time periods should therefore be handled separately when performing data analysis, in particular when calculating relevant diagnostic parameters. To this end, the boundaries between day and night must be identified in advance.
The invention described below involves methods for determining the means (averages) of physiological variables that are subject to day and night periods of the circadian variation, in particular heart rate. These methods can be used to calculate diagnostically valuable quantities for diagnosing and monitoring cardiac diseases, and to delineate the time intervals in which the patient is awake or sleeping, to thereby select data segments that are suitable for use in other data analysis methods. Furthermore, medical devices, and in particular implantable devices (e.g., cardiac pacemakers, implantable cardioverters/defibrillators, implantable devices for patient monitoring, and the like) equipped with this algorithm can tailor their functions in a patient-specific manner to the different demands in the day phase or the night phase, and thereby further improve the patient's quality of life.
From the prior art it is known to determine the respective day and night means of physiological variables at fixed points in time, for short measuring times. Day and night is phases are thereby defined on the basis of fixed time boundaries, or patient-defined protocols are used. Determining the means at fixed points in time has three disadvantages:    1. They are negatively affected by individual/temporary deviations of the rhythm that has set in. For example, measurements of the nocturnal heart rate at 3:00 a.m. are influenced by whether the particular patient has just fallen asleep, is now in a phase of deep sleep, will wake up soon, or is going to the bathroom.    2. They are based on a short measuring time. As a result, they are more susceptible to interferences of the kind described in the example above.    3. By definition, they do not provide a quantification of the circadian variation in the sense of determining sleeping times and waking times.
As an extreme example, the heart rate is measured at 3:00 a.m. when the patient wakes up briefly, and in the afternoon when the patient takes an afternoon nap. Using this approach, complete inactivity of the patient would be detected since there is little difference between the diurnal heart rate and the nocturnal heart rate. A similar problem arises, for example, when data is only analyzed in a time period defined as the nocturnal phase. This procedure discards a portion of the data that are actually available, and/or runs the risk of also integrating data from the waking phase.
The use of patient-defined protocols which rely on the patient to define diurnal, nocturnal, or intermediate periods is disadvantageous for the following reasons:    1. They are unreliable because they are dependent on the thoroughness and mental acuity of the patient.    2. They pose an imposition on the patient.